Features of the gamma-ray pulsar halo HESS J1831$-$098

TL;DR

This study characterizes the gamma-ray pulsar halo HESS J1831-098, demonstrating its similarities to the Geminga halo and exploring models to explain its slow diffusion environment and injection spectrum.

Contribution

It provides the first detailed analysis of HESS J1831-098 as a pulsar halo, applying two diffusion models to interpret observational data and infer physical properties.

Findings

HESS J1831-098 meets all criteria for a pulsar halo.

Diffusion coefficient and energy conversion efficiency are similar to Geminga.

Models with slow diffusion and modified injection profiles fit the data well.

Abstract

Gamma-ray pulsar halos are ideal indicators of cosmic-ray propagation in localized regions of the Galaxy and electron injection from pulsar wind nebulae. HESS~J1831$-$098 is a candidate pulsar halo observed by both H.E.S.S. and HAWC experiments. We adopt the flux map of the H.E.S.S. Galactic plane survey and the spectrum measurements of H.E.S.S. and \textit{Fermi}-LAT to study HESS~J1831$-$098. We find that HESS~J1831$-$098 meets all the criteria for a pulsar halo. The diffusion coefficient inside the halo and the conversion efficiency from the pulsar spin-down energy to the electron energy are both similar to the Geminga halo, a canonical pulsar halo. The injection spectrum can be well described by an exponentially-cutoff power law. However, the needed power-law term is very hard with $p\lesssim1$ if the diffusion coefficient is spatially and temporally independent. Considering the possible origins of the slow-diffusion environment, we adopt the two-zone diffusion model and the time-delayed slow-diffusion model. Both the models can interpret the H.E.S.S. and \textit{Fermi}-LAT results with a milder $p$. A modified injection time profile may have a similar effect.